Tuesday, October 16, 2007

Posterior MI Changes

The above ECG strips were taken from a 58 years old was admitted two days ago with history of chest pain that started about 30 minutes prior to admission to Emergency Department. (Click on each ECG strip to get a clearer, larger view.)

The ECF obviously show ST elevations in leads II, III and aVF, indicating that he has acute inferior wall MI.

However, leads V1 and V2 also show ST depression, and particularly in lead V2, an increase R/S ratio of >1 (previous ECG strip taken prior to this also demonstrated increased height of R but the ST depression was no so obvious as this one).

So, this patient also had a posterior wall infarct besides the inferior wall MI (inferoposterior wall MI). Isolated inferior MI can occur but more commonly inferior wall MI extends to the posterior wall, right ventricle as well as lateral MI.

He was given fibrinolytic therapy and admitted to CCU after completion of fibrinolytic in ED.

Posterior STEMI
The posterior wall of the LV is perfused by the posterior descending coronary artery, which is usually originates from the RCA (and occasionally from the left circumflex artery).
Therefore, posterior MI usually associated with inferior or lateral wall MI

Posterior MI
Standard 12 lead ECG does not directly image the posterior wall. Therefore, identifying reciprocal changes in leads V1 & V2 (leads that are 180 degrees away from posterior region) help establish diagnosis of posterior MI

ECG changes of Posterior MI
1. ST depression V1, V2 (or V3)

2. R:S ratio ≥ 1

Enlarged view of lead V2 with posterior MI changes.

ST depression in V2 may also be due to septal ischemia; but in this case R:S ratio less than 1 and R wave width less than 0.03 s

* For posterior MI R:S ratio is more reliable indicator of posterior MI compared to ST depression

However, increased R:S ratio ≥ 1 in V1 or V2 maybe due to other causes as well like:
b. RVH
c. in young children and adolescents, can be a normal variant

To detect posterior MI, posterior leads V7 – V9 may also be used.

Lead V7 – posterior axillary line
Lead V8 – inferior angle of scapula
Lead V9 – left paraspinal border

Chan TC, Brady WJ, Harrigan RA et al. ECG in Emergency Medicine and Acute Care. 1st ed. Pennsylvania: Elsevier Mosby; 2005.

Good web resources on posterior MI
1. For another clear, good picture of ECG showing posterior MI, click here
2. For a concise description on ECG changes of posterior MI, click here

1 comment:

ahmed said...

increased R/S ratio in nonspecific for posterior mi as you mentioned also it can happen in wpw syndrome but and also it will help if you have older ecg for the patient which shows normal ratio.


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